The invention relates to a press capable of pressing articles with high accuracy (within close tolerances) and at a high rate.
A typical conventional press is schematically illustrated in FIG. 7. Such a press includes a main body 100 and a stroke generating unit 110. The main body 100 comprises an upper die 103 mounted to a ram 101 and a lower die 104 mounted to a bolster 105 located on a bed 106. Pressing is performed by causing the ram 101 to make an up-and-down stroke. Reference numeral 102 is a slide guide gib for the ram 101.
The stroke generating unit 110 includes a crankshaft 111 having an eccentric member 112. The crankshaft is rotatably mounted in a bearing 113. The eccentric member 112 is connected to the ram 101 via a connecting rod 114. The crankshaft 111 is coupled with a flywheel 115 which is connected via a belt 117 to a motor 116. The drive power of the motor 116 is transmitted to the crankshaft 111 after being stored in the flywheel 115. A clutch/brake means not shown in the drawing is interposed between the flywheel 115 and the crankshaft 111.
An effective step, generally believed to improve the accuracy with which articles are produced by the press, is to reduce the shock at the start of pressing and slow the downward speed thereafter. However, this could have an adverse effect of increasing the period of each cycle of the press, resulting in a lower productivity. Also, considering technical properties peculiar to the press, it is impossible to vary the rotational frequency of the crankshaft from the flywheel side.
Therefore, various improvements have been made to a reciprocation mechanism so as to speed up most of the vertical stroke of the ram and make the ram as slow as possible at the time of pressing. But the achievements which can be obtained with such improvements of the reciprocation mechanism, are limited as will be described below.
Under these circumstances, there has been proposed a press, illustrated in FIG. 8, that is equipped with a power generating unit 120 whose crankshaft 111 can be rotated at a variable frequency. The power generating unit 120 comprises a cylinder unit 123 having a cylinder 124 and a piston 125, rotatably supported via a rod 121 in a bearing 122. The cylinder unit 123 has a piston rod 126 connected via a link rod 127 to the crankshaft 111. Pressurized oil from a pump 129 driven by a motor 128 is supplied to the cylinder unit 123 with use of an electromagnetic valve 130, etc. for controlling the speed of the piston rod 126. A flywheel is indicated by the reference number 131. With this mechanism, it would be possible to ease the shock at the start of pressing and change the pressing speed as well.
However, due to inherent properties of the cylinder, it is actually impossible to achieve high speed in one part of the stroke and low speed in another part, as well as to achieve a desired rate of change of speed in respective parts. Therefore, the press shown in FIG. 8 cannot satisfy both the requirement of a high accuracy and the requirement of a high-speed operating cycle, but only one should be selected. With this press, it is difficult to obtain a smooth stroke of the ram. Connection and automation of fittings are also difficult.
Both presses shown in FIGS. 7 and 8 generate a press forming force with use of a reciprocation mechanism. However, according to numerous analyses performed by the present inventors, the reciprocation mechanism has an inherent characteristic which seems to prevent operations at a very high accuracy. In particular, press forming starts when the connecting rod 114 is inclined with respect to the vertical axis at an angle .alpha. shown in FIG. 9 (e.g. 5 to 7 degrees). Therefore, upon application on a press forming force F, which is the vertical load, there occurs a large horizontal thrust T equal to F tan .alpha.. Thus, even if the gib 102 has a strong and tough structure, the lower surface of the ram slants and slides sideways, thereby resulting in decreased pressing accuracy. Further, the thrust load T will damage dies and products and impede a smooth stroke of the ram 101.